Single-phase, high starting torque motors, sometimes known as split-phase or capacitor-start motors, are commonly used in household appliances such as air-conditioners, washing machines and refrigerators to drive compressors and the like. Such motors incorporate a main or run winding, which is continually connected to an AC power supply during the operation of the motor, and an auxiliary or start winding which is initially connected to the power supply to generate enough starting torque to initiate rotation of the motor shaft and thereafter effectively disconnected from the motor circuit. In certain types of capacitor-start motors, two capacitors are used. A start capacitor in series with the start winding is provided to generate a phase shift between currents of the two windings while a run capacitor remains connected to the start and run windings during operation of the motor to provide a current to the start winding to increase the power factor for better efficiency.
Typically, in such single-phase motors a centrifugally operated switch or mechanical relay is provided to disconnect the start winding and start capacitor from the power supply when the motor has reached about 75% of its rated RPM. The start winding is disconnected at this point because there is enough motor torque at that speed level to overcome the increased load and to boost the motor to its rated RPM. If the start winding is disconnected too early, for example at about 50% of the motor's rated RPM, the motor experiences a breakdown torque, i.e., at 50% of its rated speed the run winding cannot continue to increase the RPM and the motor will die down to a stall.
Two types of mechanical relays often used with single-phase high starting torque motors are current activated relays and potential (voltage) activated relays. The current relay is generally limited to use only with small motors with a maximum of about one-third horsepower. This is because the energizing coil of the relay, which is in series with the motor's run winding, experiences the current of the run winding and can be damaged by high current levels. Higher horsepower motors therefore generally use potential relays because the potential relay's energizing coil is voltage activated and therefore not affected by high current surges in the run winding.
The present state of the art of most starting circuits employing potential relays is to connect the energizing coil of the relay across the start winding of the motor such that the induced voltage in the start winding will also be sensed by the relay coil. Because there is a direct relationship between the RPM of the motor and the induced voltage on the start winding, it is a simple matter to note the voltage when 75% of the motor speed is attained and disconnect the start winding and start capacitor at that point. This point at which the relay activates to disconnect the start winding from the power supply is what is generally referred to as the "pick-up" voltage. Therefore, a particular relay is chosen so that at the pick-up voltage, its coil will activate an armature and open normally closed contact points to effectively disconnect the start winding and start capacitor from the power supply.
Oftentimes a service technician must replace a defective or worn potential relay in the motor starting circuit. However, selecting a suitable replacement relay for the particular motor can be problematic. This is because when an engineer designs a motor for a specific application, such as for a refrigerator compressor, the engineer does not refer to any standard start and run winding specifications, since these specifications generally do not exist. As a result, motors of the same rated horsepower but manufactured by different companies will have a variety of different pick-up voltage points. This situation has led to confusion in the replacement market for defective and worn potential relays.
Furthermore, replacement of a relay can be a problem since manufacturers of potential relays do not generally identify the relays by their pick-up voltage points. Typically, the only numbers imprinted on a relay are for internal use by the manufacturer of the particular unit. Therefore, a service engineer who has to replace a defective potential relay usually must show the relay to the wholesaler who in turn must refer to many cross-reference charts to match the particular relay's number to a pick-up voltage, and then cross-reference that pick-up voltage to a relay manufacturer who hopefully has that relay in stock. Moreover, if there are not any identifiable numbers on the relay, the service engineer must contact the manufacturer of the unit to determine what replacement relay can be used.
All of this effort involved with what should be a simple task of replacing a relay wastes unnecessary time, increases labor costs, and ultimately results in higher repair costs for the consumer. Moreover, since each given motor from a manufacturer usually requires a different relay, a service technician must keep a large inventory of relays rated for many different pick-up voltages to guarantee that the right replacement relay is on hand for ready replacement. This also increases repair costs since many relays purchased and kept on hand are never even used.
Some effort have been devoted to designing induction motor protective circuits which include starting circuits as well. For example, U.S. Pat. No. 4,196,462 to Pohl is directed to a motor protective control circuit for an induction motor having both the start and run windings connected to the motor during its operation. As a protective measure, a mechanical relay is used to completely disconnect the power source from both windings of the motor upon excessive loading or if the start winding voltage falls outside adjustable maximum and minimum values. A timing circuit is also provided for disconnecting the power supply from the motor if the start winding voltage does not rise above the minimum cut-off value. However, since the motor does not include an auxiliary start winding which is to be disconnected from the motor after start-up, the protective control circuit is not concerned with removing the start winding once the pick-up voltage is attained.
Although mechanical relays have been used for many years in motor starting circuits and are desirable because of their relatively low cost and reliability, newer motor starting circuits are now replacing the mechanical relay with a solid-state switching device, such as a triac, to electronically switch out the start winding after a certain condition has been satisfied.
For example, U.S. Pat. No. 5,296,795 to Dropps et al. is directed to a starting system for electrical motors which uses a solid-state triac as a switching device which is serially connected to the start winding of the motor. The start winding "cut-in" and "cut-out" voltages, along with the slope of the motor speed cut-out relative to line voltage, are calibratable by changing the values of various resistors. Although this reference recognizes the need to provide a starting circuit to be used with a wide range of motor ratings, it does not contemplate the use of a device in which the "cut-out" or pick-up voltage is adjustable on site or in the field by the user. In other words, the calibration resistors initially selected and are not adjustable once installed in the starting circuit.
U.S. Pat. No. 4,804,901 to Pertessis et al. is directed to a motor starting circuit which monitors an average value in the main winding current for switching power to the start winding. A circuit is provided to generate a certain reference or threshold level based on the peak value of the main winding current. An electronic switch consisting of two triacs is provided and controlled by a comparator for switching the current on and off in the start winding based on measurements of the main winding current. The threshold generation circuit adapts to any value of current in the main winding independent of the size of the motor so that a single circuit can potentially be used for motors having various horsepowers without the need for setting a specific threshold value. However, this starting circuit is not replaceable, designed for a particular motor and is non-adjustable in the field.
Other starting circuits for induction motors use timers to disconnect the start winding and start capacitor from the power supply after a given period of time independent of the measured voltage in the run or start windings. For example, U.S. Pat. No. 4,786,850 to Chmiel is directed to a motor starting circuit with a time delay cut-out of the start winding which uses a triac and an RC timing circuit to disconnect the auxiliary start winding after a preset time delay. Likewise, U.S. Pat. No. 4,366,426 to Turlej is directed to a time-responsive switching circuit for a single-phase motor in which a triac in series with the start winding of the motor is controlled to disconnect the start winding after a predetermined time delay.
Thus, in the present age of solid-state electronics, it has been the trend in the industry to replace mechanical relay switches with electronic solid-state switches such as triacs. Although some advantages are gained with solid-state switches such as longer life and a sparkless operation that may have advantages in combustible atmospheres, several undesirable effects result. For instance, the control electronics required to operate the solid-state switch are typically complicated, require additional components and therefore increase manufacturing costs of the starting circuit resulting in a more expensive product. In addition, with solid-state switches, there can be an unwanted delay associated with the energizing of the electronic switch before it disconnects the start winding and capacitor which can cause damage to the motor. Also, an electronic switch should have a much higher rating than is necessary for a particular application because it cannot handle an excessive overload such as a locked rotor condition.